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Around the same time that the W.H.O. was hatching plans for its mega-trial, U.S. government officials were discussing how they could best invest in — and speed up — vaccine trials. Some researchers, including Dr. Fauci, advocated a design much like the W.H.O.’s.

But Moncef Slaoui, the chief adviser of Operation Warp Speed, the multiagency effort to hasten the development of coronavirus vaccines and treatments, said in a statement that such a trial would have been impractical. “If OWS had tested all vaccines under one master protocol, the operation would have had to wait months before starting and recruit 200,000 volunteers at the same time.”

In the end, the government opted for what it described as a “harmonized approach.” It would allow vaccine makers to run their own trials, but only if they used protocols that followed certain guidelines and let the National Institutes of Health test all of their volunteers in the same way. In exchange for following these rules, the companies would get to tap into to the N.I.H.’s large network of clinical testing sites, and receive major financial support for their trials. Through this program, the government has promised $10 billion to vaccine makers to date.

So far, AstraZeneca, Johnson & Johnson and Moderna have begun trials in the network. Novavax and Sanofi are expected to start their own Phase 3 studies in the next couple of months. But Pfizer, one of the front-runners, never joined the network, opting to run trials completely on its own.

If Pfizer’s results turn out well, many experts expect the company to ask the Food and Drug Administration for an emergency authorization of its vaccine, potentially for just one group of high-risk people. The company might then swiftly move to apply for a license, making it widely available.

The authorization of a vaccine will depend on how much protection the vaccine provides in the Phase 3 trial — what scientists refer to as its efficacy. In June, the F.D.A. set 50 percent efficacy as the target for a coronavirus vaccine.